Are you aware of any websites that have tried to implement payments, but failed or chose not to because they couldn't verify which users have paid? It's an incredibly easy problem to solve without WEI.
Nah, let jfoutz pay the fraction of a penny and get no ads. Content producer gets paid, and I get to read the article.
The site can choose, based on attested properties to send to an ad network, or just take the money from the user.
There are other ways to do it, but this makes it a standard.